General code improvements

Already squashed commits:

- Improving commands handling in rake preview task
- Fix flags disposition in README
- Removed build_test task in favor of install'provided by bundler. More details: https://github.com/carlhuda/bundler/blob/master/lib/bundler/gem_helper.rb
- Defining a RSpec rake task
- Simple stylish improvements in gemspec
- Better handling of spec's tmpdir
- Updated dependencies
- Better specs and test ecosystem
- Simple quotes instead double quotes